Various electronic apparatuses, such as radio base station apparatuses in radio communication systems, have a plurality of boards therein, and electronic devices such as large scale integrated circuits (LSIs) are implemented on the boards. In such an electronic apparatus, transmission of digital signals in each of the boards or between the boards is realized by serial transmission or parallel transmission of square wave signals using various forms. In a parallel transmission scheme, various types of circuit conditions associated with widening of the signal frequency band are relatively loose. However, because the number of pins of devices and the number of control signals are increased, it is difficult to satisfy requests for reduction in power consumption, reduction in size, and so forth. Examples of the various types of circuit conditions include conditions for filters for reducing high-frequency signals, conditions for termination, conditions for reflection, and so forth. On the other hand, in a serial transmission scheme, the number of signals is reduced, compared with the number of signals in the parallel transmission scheme. However, as compensation for reduction of the number of signals, because the transmission speed is increased, the signal frequency band is widened. Accordingly, in order to reduce high-frequency signals that occur, a complicated correction circuit is used. Furthermore, an impedance at a terminal of a transmission path is strictly controlled. For example, for high-frequency signals that occur because a signal passes along a transmission path, a circuit that corrects the amplitudes or delays of the high-frequency signals in advance on a transmission side is used. When the high-frequency signals are not completely removed, a correction circuit that performs correction in consideration of reflection of the high-frequency signals at the terminal is used concurrently with the above-mentioned circuit. Furthermore, in order to terminate a wideband signal including a direct-current element, typically, a resister that is suitable for a characteristic impedance of each of the boards is used. A current that is determined based on the value of the register always flows. This leads to increase in current in the entire electronic apparatus having a plurality of signal lines in each of the boards and between the boards. One example of the value of the resistor that is suitable for a characteristic impedance of each of the boards is 50 Ω.
Furthermore, in SERializer/DESerializer (SerDes) that is a high-speed serial transmission scheme which is common at present, using an 8b/10b code or the like, continuous codes are prevented from being generated for a specified number or more of clocks, thereby reducing occurrence of high-frequency signals to some degree. However, in the SerDes, because a redundancy signal is added, the amount of data is reduced for the transmission speed.
Additionally, a scheme has been proposed, in which data is transmitted by adding information concerning the data to a carrier wave using phase shift keying (PSK) (for example, see Japanese Laid-Open Patent Publication No. 8-125703). However, in the scheme using a carrier wave, the frequency of the carrier wave does not match to the transmission speed, and points at which the phase is changed are not in synchronization with states of the carrier wave. Accordingly, a complicated demodulation circuit such as a demodulation circuit for radio signals is desired. Thus, it is difficult to reduce the size, cost, and power consumption of the electronic apparatus.